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OSPF - Operation

How it all works

• Internet model

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AS X AS Y AS Z

iBGP iBGP iBGP

IGP IGP IGP

eBGP eBGP

Barry Greene & Philip Smith “Cisco ISP Essentials”

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Recap

• In ISP networks, IGPs– Allow routers within an AS to learn about each other

– Carry next-hop reachability info• Carries infrastructure info (loopbacks & ptp)

• NOT customer routes!

– scalability and fast convergence• Hence, minimise the number of prefixes carried in IGP!

• BGP– Carries customer prefixes

– Exchanges network info with other networks• Carries internet route across the AS

OSPF - Intro

• Link-state protocol– SPF algorithm

– Protocol number (89) – runs on top of IP

– Only sends triggered updates

– Supports hierarchical routing (multi-area) – scalability

• OSPFv2 (RFC2328), OSPFv3 (RFC5340), OSPFv3 AF (RFC5838)

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OSPF MessageOSPF HeaderIP Header

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LSAs

R2 R1 LSDB (Topology)LSAs

SPF Tree

Best Paths

Routing Table

Dijk

stra

’s S

PF

Link State Operation

R2

R3

R1

R4

Link State Operation

• Each link-state router learns about its links and connected networks– builds a link state packet – LSP (LSAs for each link)

• Sends out Hellos for neighbor discovery– To establish adjacency

• Floods LSP to all its neighbors– Stores all LSPs learned from its neighbors in a LSDB, and floods

to other neighbors

– Eventually all routers receive all LSPs• same view of the network!

• Computes the best path to each destination – using the SPF algorithm (SPF tree)

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Shortest Path First (SPF) Tree

• Every router in an OSPF network maintains an identical topology database

• Router places itself at the root of SPF tree when calculating the best path

Best path selection

• Lowest cumulative cost = best path

• Load balances over equal cost paths

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FE

FE

FE

GE

1

10

10

10

FE

FE

GE

GE

1

10

10

1

OSPF Metric

• Uses link/path cost as metric

• Generally, inversely proportional to the link BW– Higher the BW lower the cost

• configurable

– The reference BW is generally 100Mbps (FE)• interfaces bigger than a FE would have a cost of 1

– For more granularity/accuracy of cost calculation• change reference BW for bigger links (all OSPF routers)

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Cost =𝒓𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆 𝒃𝒘

𝒊𝒏𝒕𝒆𝒓𝒇𝒂𝒄𝒆 𝒃𝒘(𝒃𝒑𝒔)

IOS:router ospf/v3 <process-id>

auto-cost reference-bandwidth <Mbps>

Junos:set protocols ospf/3 reference-bandwidth <Gbps>

Router ID

• Uniquely identifies a link-state router– 4-byte Router ID

• Either:

– Explicitly configured =>

– Else, the highest/lowest IPv4 address of any active loopback interface

– If no loopbacks, the highest/lowest IPv4 address of any active physical interface

– ** Loopbacks preferred!• Why??

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IOS:router-id <4-byte>

Junos:set routing-options router-id <4-byte>

OSPF Packets

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Version Type Length

Router-ID

Area-ID

Checksum AuType

Authentication

Authentication

Type Description

1 Hello

2 Database Description

3 Link State Request

4 Link State Update

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Link State

Acknowledgement

OSPF Header OSPF Message

OSPF Packets

• Hello packet

– Initially exchanged for neighbor discovery, and

– Later periodically to maintain adjacency• Hello/Dead interval

– Hello packets contain:• Router ID (sender’s)

• Area ID *

• Hello and Dead interval *

• Neighbors (list/router-id of neighbors - valid Hellos received)

• Network mask *

• DR/BDR identity (IP)

• Authentication (if enabled) *

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OSPF Packets

• DBD packet

– To sync LSDBs

– Summary of local LSDB• List of LSAs (headers) with sequence number

– DBD exchange uses a poll-response paradigm• Master sends DBD (polls); Slave acks with its own DBD (responds)

– The router with higher Router ID - Master

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OSPF Packets

• LSR packet– During the sync process, if the local router finds its LSDB is

out of date (older LSAs than those received in the DBD –seq#)

– Sends a LSR for the missing/newer LSAs• Each LSA requested is identified by LS type, link state ID, and the router

that advertised the LSA

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OSPF Packets

• LSU packet– Link state info is propagated through LSAs

– LSAs are advertised in LSU packet to neighbors• Flooded reliably (ack) throughout the nw

• Initially (after adjacency is built), all LSAs in local LSDB

• Later, LSUs are only sent during a topology change

• Also in response to LSRs

– LSUs contain• # of LSAs

• List of LSAs (single or multiple)

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OSPF Packets

• LSAck packet– OSPF routers need to acknowledge receipt of each LSA

• LSAck

– Dataless packet

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Neighbor States

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+----+

|Down|

+----+

+----+

|Init|

+----+

+-----+

|2-way|

+-----+

Hello received (no local Router-ID)

2-way received (local Router-ID seen)

DR/BDR selected from the “neighbor” list

+-------+

|ExStart|

+-------+

Master-Slave decision + DBD Seq#

(higher router ID)+--------+

|Exchange|

+--------+

Exchange DBD (local LSDB)

+--------+

|Loading |

+--------+

+----+

|Full|

+----+

LSRs sent to neighbor Adjacency estb

(inc Router LSAs &

Network LSAs)

Y - more recent LSAs req?N

Scaling - Hierarchy

• With single area OSPF, as network grows:– Larger LSDB and routing table

– Frequent LSA flooding and SPF compute

• Hence, two-level hierarchy– LSA flooding contained within areas

– Only summary routes exchanged between areas• Through the backbone

Area 0

Area 1Area 3

10.10.10.0/26

10.10.10.64/26

10.10.10.128/26

10.10.10.192/26

2001:db8::/64

2001:db8:0:1::/64

2001:db8:0:2::/642001:db8:0:3::/64

Area 2

ABRABR ABR

Virtual links

• OSPF requires regular areas to be connected to the backbone (Area-0)– Inter-area routes propagated through the backbone

• Virtual links allow regular areas to connect (logically) to the backbone– Physically not feasible

Area 0

Area 1

Area 3ABR ABRVirtual link

OSPF Network Types

• Point-to-point– Can ONLY have one neighbor

connected on the link

– LSUs sent to ”all OSPF routers” multicast• 224.0.0.5/FF02::5

• Broadcast (multi-access) – Could have more than one

neighbor connected on the link• Ethernet links

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Broadcast NW issues

• Number of adjacencies– # of Adj = n(n-1)/2;

• Extensive LSA flooding– Initially, the whole LSDB

• LSAck too

– Periodic hellos for adjacencies

– Triggered updates• During topology changes, each router will

send LSUs to neighbors - contains the same info

• LSAck too

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LSALSA

LSA LSALSA

LSALSALSA

LSA

DR/BDR

• Hence, OSPF elects a Designated and Backup Designated router for broadcast networks– Adjacencies only formed with DR and BDR

– LSAs sent only to DR (BDR listens)• 224.0.0.6/FF02::6

– DR floods to others• 224.0.0.5/FF02::5

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DR BDR

DR/BDR Election

• Uses the Hello protocol (Rtr Priority)– highest OSPF interface priority – DR

• Next highest priority – BDR

– Configurable:

– Else, highest router ID – DR• Next highest - BDR

– Recommended:• configure higher priority for routers meant to be DR and BDR!

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IOS:(config-if)#ip/ipv6 ospf priority <0-255>

Junos:set protocols ospf/3 area <area-id> interface <id> priority <0-255>

LSA Types

• LSA Header

– Age: time since LSA was generated

– Link state ID: identifies what the LSA is carrying

– Advertising Router: Router ID of the router originating the LSA

– Seq#: indicates newness of the LSA

– Checksum: of the LSA content except age

– Length: of LSA (includes 20 byte header)

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Type Link-state ID

1

Originating router’s router

ID

2 Interface IP of DR

3 Network address

4 ASBR’s router ID

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The external network

address

LS age Options LS Type

Link State ID

Advertising Router

LS Sequence#

Checksum Length

Type Description

1 Router LSA

2 Network LSA

3 Summary LSA (ABR)

4 Summary LSA (ASBR)

5 AS-external LSA

LSA Types

• Type-1 (Router LSA)– router’s connected (active) links/interfaces and metrics

– flooded within the area (does not cross ABR)

– Identified by router ID of originating router

• Type-2 (Network LSA)– broadcast/multi-access networks

• generated by DR

– describes routers connected to the broadcast segment• Adjacent to the DR, including itself

– flooded within the area

– Identified by DR’s interface IP

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LSA Types

• Type-3 (Summary LSA - ABR)– Inter-area routes

• Allows condensation at the are borders

– Originated by the ABR• ABRs store LSAs from each area in a separate LSDB

• Generates a Type 3 for each subnet in the area

– Floods to the backbone• Propagated to other areas

– Identified by the subnets carried in the LSA

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LSA Types

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Type 1

Area10

Area0Backbone

Area20

LSA Types

• Type-4 (Summary LSA - ASBR)– To identify the ASBR

• Route/path to the ASBR, to forward traffic destined for nws outside the domain

– Originated by the ABR• When a ABR receives Type-1 LSA with the E-bit set, generates a Type-4

LSA

– Floods to the backbone• Propagated to other areas

– Identified by router ID of the ASBR

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LSA Types

• Type-5 (AS-external LSA)– To advertise external routes into OSPF

• Destinations outside the domain

– Originated by the ASBR• Generates a Type-5 for each external route

– Floods to the area it belongs• Propagated unaltered to other areas via backbone by ABR of the area

– Identified by the external subnet carried in the LSA

• Note: DO NOT redistribute external routes into OSPF!– Floods unaltered throughout the network – convergence??

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LSA Types

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Type5

Area10

Area0Backbone

Area20

Type4

Non-OSPF

LSA Types

• Opaque LSAs– To advertise optional router capabilities

• Protocol extensibility

• RFC5250

– Ex: Router Information Opaque LSA (RFC7770)• SR capabilities

– Flooding scope depends on the type• 9- link

• 10 – area

• 11 - domain

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Best path compute

• Intra-area routes– Lowest cost to the each nw within the area

• Inter-area routes– Type3 (summary LSA-ABR) includes cost to each network– Best path = lowest (cost to ABR + cost in Type3 LSA)

• External routes (E-bit defines the metric type – E2>E1)

– If not set (0), Type-1 ext metric (E1): cumulative as the route

gets propagated through the domain (more than one ASBR)

• Seed metric + cost to ASBR

– If set (1), Type-2 ext metric (E2): same throughout the domain

(only one ASBR)

• Seed metric

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OSPFv3 – RFC5340

• To carry IPv6 in OSPF– Router ID, Area ID and link state ID still 32-bit

– OSPFv3 uses link-local address for adjacency• All OSPFv3 packets are sourced using link-local

– LSUs are sent to the IPv6 multicast• FF02::5 (all OSPF routers) and FF02::6 (DR/BDR)

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OSPFv3 – New LSAs

• Type-8 (Link LSA)– Advertise local router’s link-local to other routers on the link,

and list of v6 prefixes associated with link

– Not flooded beyond the link (link-scope)

– Link state ID is the interface ID on the link

• Type-9 (Intra-Area-Prefix LSA)– Either advertises v6 prefixes associated

• With a router (directly connected interfaces) – references Router/Type-1 LSA, OR

• With a broadcast segment (Network/Type-2 LSA)

– Flooded within the area (area-scope)

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OSPFv3 – Renamed LSAs

• Type-3 (Inter-Area-Prefix LSA)– Equivalent to Type-3 summary LSA in OSPFv2

• Describes routes to v6 prefixes within an area

– Originated by ABR• Generates a Type-3 for each IPv6 prefix

– Flood to the backbone for propagation to other areas

• Type-4 Summary (Inter-Area-Router LSA)– Equivalent to Type-4 summary (ASBR) in OSPFv2

• Route(s) to ASBR

– Originated by ABR

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Passive interface

• When OSPF is configured/enabled on an interface (or for a subnet)– router will try to discover neighbors on that interface/within

the subnet

– We can disable sending OSPF packets on those interfaces where we know there will be no neighbors• While still advertising the network in OSPF

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IOS:ipv6 router ospf <process-id>

router ospf <process-id>

passive-interface default

no passive-interface <interface-id>

IOS:ipv6 router ospf <process-id>

router ospf <process-id>

passive-interface <interface-id>

Junos:set protocols ospf/3 area <area-id> interface <int-id> passive

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